1. Field of the Invention
This invention relates to filtering devices employing continuous filters to filter high temperature flowable thermoplastic or thermoset materials.
2. Description of the Prior Art
In the conventional filtering devices of the above-mentioned type, a housing defines a filter passage through which the thermoplastic material is caused to flow at an elevated temperature and pressure. The housing has entry and exit ports flanking the filter passage. A continuous filter is moved through the entry port, across the filter passage, and out through the exit port, with that portion of the filter extending across the filter passage being arranged to filter the thermoplastic material flowing therethrough.
Various arrangements have been proposed for moving the filter through the filter passage. In one such arrangement, as disclosed for example in U.S. Pat. No. 3,971,721, an external device is employed to mechanically grip and pull the filter through the device. Although such drives operate in a satisfactory manner, they are expensive and they occupy considerable space, thus making the filtering devices bulky and difficult to position in the confined spaces which are often found along commercial production lines.
U.S. Pat. No. 3,471,017 discloses another prior art arrangement where a solidified plug of the thermoplastic material is formed in the exit port. The plug is bonded to the filter and is forced out through the exit port by the pressure of the flowable material in the filter passage. The drawback with this type of arrangement is that once the plug is formed, the temperature at the exit port must be precisely controlled to within.+-.a few degrees in order to reheat the solidified material just enough to achieve carefully controlled plug movement. Insufficient heating stalls the plug in the exit port, while excessive heating can result in the plug being torn away from the filter and in extreme cases, being blown out of the exit port by the pressure of the flowable material in the filter passage. Moreover, the required temperature ranges vary depending on the material being filtered, thus making it difficult to operate this type of device on a commercial scale.
Other prior art arrangements of the type disclosed in U.S. Pat. No. 4,021,346 attempt to combine the plug concept with mechanical external drives. Here again, close control must be exercised over the temperature of the plugs, and the external drives are expensive and bulky.